1. Field of the Invention
The present invention relates to an apparatus for combining a plurality of video signals into a single video signal.
2. Description of the Related Art
It has heretofore been customary for broadcasting stations or the like to use a video signal processor having combiners for mixing or combining a plurality of television video signals that have been given special effects by special effects systems (DME), for example.
For example, one special effects system generates an image which looks like a three-dimensional curved surface with an input video signal applied thereto. Such a special effects system is proposed in U.S. Pat. No. 4,965,844, for example.
U.S. Pat. No. 4,488,169, for example, discloses a so-called chroma key system which combines two video signals, i.e., a foreground signal and a background signal, into a single video signal based on a key signal that is extracted from the foreground signal.
Video signal processors employing a certain number of combiners will be described in detail below with reference to FIGS. 1 through 5 of the accompanying drawings.
In FIG. 1, a video signal processor includes special effects systems (DME) 1, 2, 3 for processing signals to given them special effects. Signals (video, key, and depth signals) which have been given special effects by the DMEs 1, 2 are supplied to a 2-ch combiner 4, and signals (video, key, and depth signals) that have been given special effects by the DME 3 are supplied to a 2-ch combiner 5.
The 2-ch combiner 4 combines the video signals supplied respectively from the DMEs 1, 2 into a single video signal based on the depth information and the key signal, for example. The produced single video signal is supplied, together with the key signal and the depth signal, to the 2-ch combiner 5.
The 2-ch combiner 5 combines the video signal supplied from the 2-ch combiner 4 and the video signal supplied from the DME 3 into a single video signal based on the depth information and the key signal. The produced video signal is supplied through an output terminal 6 to a television monitor (not shown), for example, which displays an image on its screen.
Therefore, the video signal processor can combine a plurality of video signals based on the depth and key signals for smoothing edges and points of intersection.
The combining process of the 2-ch combiners will be described below with reference to FIG. 2. When images A, B intersect with each other as shown in FIG. 2, the images A, B are mixed with respect to an edge a using a key signal Ka of the image A. The mixing of the images A, B with respect to the edge a is represented by the following formula (1): EQU Ka.multidot.A+(1-Ka).multidot.B(0.ltoreq.Ka.ltoreq.1) (1).
With respect to an edge b, the images A, B are mixed using a key signal Kb of the image B. The mixing of the images A, B with respect to the edge b is represented by the following formula (2): EQU Kb.multidot.B+(1-Kb).multidot.A(0.ltoreq.Kb.ltoreq.1) (2).
With respect to an edge c, an image (indicated by F) and a background BG are mixed using a combined key Kbg of the key signals Ka, Kb&gt;. The mixing of the image F and the background BG with respect to the edge c is represented by the formula (3): EQU Kbg.multidot.F+(1-Kbg).multidot.BG}Kbg=1-(1-Ka)(1-Kb)} (3).
With respect to an edge d, a priority signal (a type of key signal) Zab is generated using a depth signal Za of the image A and a depth signal Zb of the image B, and the images A, B are mixing using the priority signal Zab. The mixing of the images A, B with respect to the edge d is represented by the following formula (4): EQU Zab.multidot.A+(1-Zab).multidot.B (4).
By thus mixing the image signals using the key signals and the depth signals, all the edges can be smoothed.
The mixing of three channels or more with two or more 2-channel combiners that are connected in cascade as shown in FIG. 1 will be described below with reference to FIGS. 1 and 3A, 3B.
In FIGS. 3A, A, B, and C represent images, respectively. If the output signal from the DME 1 represents the image A, the output signal from the DME 2 represents the image B, and the output signal from the DME 3 represents the image C, then the images A, B are combined by the 2-ch combiner 4, and the output signal from the 2-ch combiner 4 and the image C are combined by the 2-ch combiner 5. The key and depth signals are similarly combined into a single key signal and a single depth signal.
At this time, as shown in FIG. 3B, the images A, B that are closer to a viewpoint p are mixed with each other, and thereafter the image C that is remotest from the viewpoint p is mixed. Therefore, an output image has no problem.
If the output signal from the DME 1 represents the image A, the output signal from the DME 2 represents the image C, and the output signal from the DME 3 represents the image B, then the images A, C are combined by the 2-ch combiner 4, and the output signal from the 2-ch combiner 4 and the image B are combined by the 2-ch combiner 5.
At this time, as shown in FIG. 3B, the image A that is closest to the viewpoint p and the image C that is remotest from the viewpoint p are mixed with each other, and thereafter the image B that is second closest to the viewpoint p is mixed. This poses a problem to an edge of an output image because the order of mixing of the images and the priority of the images (the closeness of the images to the viewpoint or the relationship of the levels of the images) do not match each other.
A degradation of the image quality due to such a mismatch between the order of mixing of the images and the priority of the images will be described in greater detail with reference to FIGS. 4A and 4B.
FIG. 4A illustrates an output image which is produced by mixing the images A, C and then mixing the mixed image and the image B. As shown in FIG. 4A, the image C which should not appear between the images A, B shows as a dithering area between the images A, B.
One way of preventing the image from being degraded as shown in FIG. 4A is not to mix the images A, C. However, such a solution also creates a dithering area along the edge between the images A, B as shown in FIG. 4B.
The image degradations shown in FIGS. 4A and 4B may be avoided by mixing the images A, B, C in the descending or ascending order of priority. However, in the system composed of the DMEs and the combiners, unlike switchers, the priority of each image varies from pixel to pixel, making it impossible to obtain good image quality in all positions on the displayed screen image.
For example, a problem occurs if the image A is semitransparent and the image B is opaque. More specifically, if the images A, C are mixed first, then the image C shows which should not appear because the image B is opaque. This problem is the same as the edge problem.
A solution to the above problem is a cascade system which take into account image priority as shown in FIG. 5.
In FIG. 5, the cascade system has input terminals 7, 8, 9 which are supplied with video, key, and depth signals. The video, key, and depth signals supplied to the input terminals 7, 8, 9 are supplied to a matrix switcher 10. The depth signals from the respective input terminals 7, 8, 9 are also supplied to a priority determining circuit 11.
The priority determining circuit 11 supplies a control signal to the matrix switcher 10 based on the depth signals that are supplied from the input terminals 7, 8, 9.
Based on the control signal supplied from the priority determining circuit 11, the matrix switcher 10 supplies two video signals of higher (lower) priority, among the video signals from the input terminals 7, 8, 9, to a combiner 12, and also supplies a video signal of lowest (highest) priority to a combiner 13.
Therefore, the combiner 12 combines the two video signals of higher (lower) priority and produces a combined output signal, which is then combined with the video signal of lowest (highest) priority by the combiner 13. Consequently, no image degradation occurs in the image combined by the cascade system shown in FIG. 5.
It is assumed that the video, key, and depth signals supplied to the input terminal 7 are indicated by Va, Ka, Za, respectively, the video, key, and depth signals supplied to the input terminal 8 are indicated by Vb, Kb, Zb, respectively, the video, key, and depth signals supplied to the input terminal 9 are indicated by Vc, Kc, Zc, respectively, the depth signals Za, Zb, Zc have a priority relationship: Za&gt;Zb&gt;Zc, the matrix switcher 10 has input terminals I1, I2, I3 arranged in the order named from above, and output terminals 01, 02, 03 arranged in the order named from above. Now, the matrix switcher 10 effects switching on the input and output terminals as follows: EQU when Za.ltoreq.Zb.ltoreq.Zc, I1=O1, 12=O2, 13=O3, EQU when Za.ltoreq.Zc.ltoreq.Zb, I1=O1, I3=O2, I2=O3, EQU when Zb.ltoreq.Za.ltoreq.Zc, I2=O1, I1=O2, I3=O3, EQU when Zb.ltoreq.Zc.ltoreq.Za, I1=O1, I3=O2, I2=O3, EQU when Zc.ltoreq.Za.ltoreq.Zb, I2=O1, I1=O2, I3=O3, and EQU when Zc.ltoreq.Zb.ltoreq.Za, I1=O1, I2=O2, I3=O3 (5)
The above process can be achieved by placing the matrix switcher 10 in front of the cascaded combiners 12, 13, and is effective to preventing the above image degradation.
However, if the number of channels is increased, then the circuit scale of the matrix switcher 10 is also increased.
There has also been proposed a process of mixing 2-ch images simultaneously rather than with the cascaded arrangement.
An output image signal Vobg produced by mixing the 2-ch images according to such a mixing process is indicated by the following equation (6): ##EQU1## where 0.ltoreq.Zab.ltoreq.1, Zab represents the priority of the images A, B, indicating that the image B is of a higher level when Zab=1 and the image A is of a higher level when Zab=1. The key signal Ka of the image A is 0 or more, and the key signal Kb of the image B is 1 or less.
An output signal V0 produced when the mixed image is not outputted is indicated by the following equation (7): ##EQU2## The combined key signal Kbg is indicated by Kbg=1-(1-Ka)(1-Kb). When the 2-ch images are mixed simultaneously, therefore, the output image is prevented from being degraded.
When 3-ch images are mixed with each other, an output image signal Vobg is produced as indicated by the following equation (8): ##EQU3##
The combined key signal Kbg is indicated by Kbg=1-(1-Ka)(1-Kb)(1-Kc). Similarly, 4-ch images can be mixed without output image degradations.
The applicant has previously proposed a special effects system for transmitting depth information using a serial transmission path which has the same specifications as those of a serial transmission line for transmitting digital signals (see, for example, U.S. patent application Ser. No. 07/798,153).
According to the above mixing processes, the amount of calculations increases in proportion to {n(n-1)} for mixing images in n channels. For example, the amount of calculations for mixing images in three channels is three times the amount of calculations for mixing images in two channels, and the amount of calculations for mixing images in four channels is six times the amount of calculations for mixing images in two channels. As the number of channels increases, therefore, the amount of calculations required increases, requiring the overall system to be of a complex and large circuit arrangement.